The present invention relates to a toroidal-type continuously variable transmission for use as an automatic transmission for a vehicle and, particularly, to a toroidal-type continuously variable transmission in which the durability of component parts thereof, that is, disks and power rollers is enhanced.
Conventionally, as a toroidal-type continuously variable transmission, there is known a transmission which has such a structure as shown in FIG. 1.
Within a housing (not shown), an input side disk 1 and an output side disk 2 are disposed on an input shaft 3 which is rotatably supported in the interior of the housing, in such a manner that they are concentric with and opposed to each other. The input shaft 3 is penetrated through the axial core portion of a toroidal transmission part including the input side and output side disks 1 and 2. A loading cam 4 is disposed on one end of the input shaft 3. The loading cam 4 is structured such that it can transmit the power (rotational force) of the input shaft 3 to the input side disk 1 through a cam roller 5. The input side and output side disks 1 and 2 are concentric with each other with their respective inner surfaces opposed to each other. The input side and output side disks 1 and 2 have substantially the same shape, while these opposed surfaces are formed as toroidal surfaces which cooperate together in forming a substantially semicircle.
Within a toroidal cavity defined by the toroidal surfaces of the input side and output side disks 1 and 2, a pair of power roller bearings 6 and 7 are disposed in such a manner that they are contacted with the input side and output side disks 1 and 2. By the way, the power roller bearing 6 is composed of a power roller 6a (which corresponds to an inner ring forming the power roller bearing 6) which is allowed to roll on the toroidal surfaces of the input side and output side disks 1 and 2, an outer ring 6b, and a plurality of rolling bodies (steel balls) 6c. On the other hand, the power roller bearing 7 is composed of a power roller 7a (which corresponds to an inner ring forming the power roller bearing 7) which is allowed to roll on the toroidal surfaces of the input side and output side disks 1 and 2, an outer ring 7b, and a plurality of rolling bodies (steel balls) 7c. 
That is, the power roller 6a serves also as an inner ring which is a component of the power roller bearing 6, while the power roller 7a serves also as an inner ring which is a component of the power roller bearing 7. In this structure, the power roller 6a is mounted pivotally and rotatably on a trunnion 10 through a pivot shaft 8, the outer ring 6b and the plurality of rolling bodies 6c on the trunnion 10 swinging about a support shaft 20 disposed at a position which is perpendicular to an axial line of the input shaft 3 and does not intersect the axial line thereof (hereinafter, this physical relationship is referred as xe2x80x3 a twisted position. The power roller 6a includes a peripheral surface which is formed as a spherical-shaped convex surface. The power roller 6a is supported in such a manner that it is able to inclinedly roll about shift shafts respectively supported on the trunnions and serving as the centers of the toroidal surfaces of the input side and output side disks 1 and 2. On the other hand, the power roller 7a is mounted pivotally and rotatably on a trunnion 11 through a pivot shaft 9, the outer ring 7b and the plurality of rolling bodies 7c, while the trunnion 11 can be oscillated about the pivot shaft 9 disposed at a twisted position with respect to the input shaft 3. The power roller 7a includes a peripheral surface which is formed as a spherical-shaped convex surface. The power roller 7a is supported in such a manner that it is able to roll inclinedly about shift shafts 0 respectively supported on the trunnions and serving as the centers of the toroidal surfaces of the input side and output side disks 1 and 2. And, to the contact surfaces of the input side and output side disks 1 and 2 with respect to the power rollers 6a and 7a, there is supplied lubricating oil which is large in viscous frictional resistance. Power input to the input side disk 1 can be transmitted to the output side disk 2 through the lubricating oil film, power roller 6a and power roller 7a. 
By the way, the input side and output side disks 1 and 2 are set independent of the input shaft 3 through a needle roller 12 (that is, they are not influenced directly by the power of the rotary shaft or input shaft 3). On the output side disk 2, there is disposed an output shaft 14 which is not only arranged parallel to the input shaft 3 but also rotatably supported by the housing (not shown) through angular bearings 13. In this toroidal-type continuously variable transmission 20, the power of the input shaft 3 is transmitted to the loading cam 4. In case where the loading cam 4 is rotated due to such power transmission, the power of the loading cam 4 produced by the rotation thereof is transmitted through the cam roller 5 to the input side disk 1, thereby causing the input side disk 1 to rotate. Further, the power of the input side disk 1 produced by the rotation thereof is then transmitted through the power rollers 6a and 7a to the output side disk 2. As a result of this, the output side disk 2 is rotated integrally with the output shaft 14.
In the case of transmission, the trunnions 10 and 11 are moved by a slight distance in the directions of the shift shafts 0, respectively. That is, due to the movements of the trunnions 10 and 11 in the axial direction thereof, the intersection between the rotation axes of the power rollers 6a, 7a and the axes of the input side and output side disks 1 and 2 are shifted slightly from each other. As a result of this, the balance between the rotational peripheral speeds of the power rollers 6a, 7a and the rotational peripheral speed of the input side disk 1 is lost. In addition, due to the component of the rotation drive force of the input side disk 1, the power rollers 6a, 7a are inclinedly rotated around the shift shafts 0, respectively. Therefore, the power rollers 6a, 7a are inclinedly rotated on the curved surfaces of the input side and output side disks 1 and 2, with the result that the speed ratio is changed, thereby accelerating or decelerating the speed of the vehicle.
As a toroidal-type continuously variable transmission having the above structure, for example, there is known a conventional transmission which is disclosed in Japanese Utility Model Examined Publication No. Hei. 2-49411U. Also, as the examples of the above-mentioned input side disk, output side disk and power roller bearings, there are conventionally known disks and bearings which, as set forth in [NASA Technical note NASA ATN D-8362], use AISI52100 (JIS SUJ2 which corresponds to a high carbon chromium bearing). Further, disks and bearings which, as disclosed in Japanese Patent Unexamined Publication No. Hei. 9-79336, are produced by carbonitriding SCM420, that is, steel for machine-structural purposes containing Cr.
In the above-mentioned conventional toroidal-type continuously variable transmission, when it is driven, there are produced high contact pressures respectively between the input side disk and power roller bearings, between the output side disk and power roller bearings, and between the power roller inner and outer rings and rolling bodies. These high contact pressures give rise to the shortened rolling fatigue lives of the raceway surfaces. Also, an increase in the temperatures of the contact surfaces due to generation of heat in the lubricating oil (traction oil) flowing through between the input side and output side disks and power roller bearings as well as through between the power roller raceway surfaces and rolling bodies gives rise to the lowered surface hardness of the disks and bearings, thereby shortening the rolling fatigue lives of the disks and bearings.
In view of the above circumstances, for the purpose of enhancing the rolling fatigue lives of the disks and power roller bearings shortened due to the above-mentioned high contact pressures and high temperatures, conventionally, the input side and output side disks and the inner rings or outer rings of the power roller bearings are carburized or carbonitrided, to thereby prevent the reduction in the high temperature hardness of the surfaces thereof. To obtain sufficient surface hardness, it is necessary to enhance the carbon concentration and nitrogen concentration of the neighboring portions of the surfaces sufficiently. Further, in the above-mentioned conventional toroidal-type continuously variable transmission, there is a fear that there can be generated a high shearing stress in the interior portions of the rolling bodies and thus there can occur cracks at and from the neighboring portions of the maximum shearing stress positions, thereby causing the raceway surfaces to exfoliate. In view of this, in Japanese Patent Unexamined Publication No. Hei. 7-71555, there is provided an effective hardened layer having a depth in the range of 2.0 mm-4.0 mm, thereby obtaining a life extension effect.
Also, in Japanese Patent Unexamined Publication No. Hei. 9-79336, the material hardness at the maximum shearing stress position is set at Hv700 or more, thereby securing a life extension effect. To obtain the above life extension effects, it is necessary to enforce a carbonizing treatment or a carbonitriding treatment on the disks and bearings for a very long time but this raises a problem that the costs necessary for the thermal treatments are large. Further, to secure sufficient quenching hardness, it is necessary to use quenching oil which has a good quenching characteristic: that is, this arises a problem that the material is greatly deformed when it is quenched.
The present invention aims at eliminating the above drawbacks found in the conventional toroidal-type continuously variable transmissions. Accordingly, it is an object of the invention to provide a toroidal-type continuously variable transmission in which a thrust rolling bearing is made of high carbon chromium bearing steel and is carbonitrided in the surface thereof to thereby provide sufficient durability when compared with the conventional toroidal type continuously variable transmissions.
In attaining the above object, according to the invention, there is provided a toroidal-type continuously variable transmission, comprising: an input shaft rotatably supported; an input side disk rotatable together with the input shaft and including an inner surface formed as a concave surface having an arc-shaped section; an output side disk including an inner surface formed as a concave surface having an arc-shaped section, the output side disk being disposed concentrically with the input side disk with the inner surface thereof opposed to the inner surface of the input side disk; a plurality of trunnions respectively oscillatable about their associated pivot shafts disposed at twisted positions to the input shaft; a plurality of shift shafts respectively supported on the trunnions; a plurality of power rollers each including a peripheral surface formed as a spherical-shaped convex surface, the power rollers being rotatably supported on the peripheries of the shift shafts while they are disposed on the inner surfaces of the trunnions and also are interposed between and held by the input side and output side disks; and, a plurality of thrust rolling bearings respectively interposed between the outer peripheral surfaces of the power rollers and the inner surfaces of the trunnions for supporting thrust loads to be applied to the power rollers, wherein each of the thrust rolling bearings is made of high carbon chromium steel and is carbonitrided.
In the preferable embodiment of the present invention, each of the thrust rolling bearings may include:
an inner ring raceway formed in the outer peripheral surface of the power roller; and
an outer ring disposed inside the trunnion and having an outer ring raceway,
wherein the outer ring is made of high carbon chromium steel and is carbonitrided.
According to a second aspect of the invention, each thrust rolling bearing, preferably, may be structured such that the surface carbon concentration thereof is set in the range of 0.95-1.4% and the surface nitrogen concentration thereof is set in the range of 0.05-0.4%.
Also, according to a third aspect of the invention, each thrust rolling bearing, preferably, may be structured such that the surface hardness thereof is set at Hv650 or more.
Further, according to a fourth aspect of the invention, each thrust rolling bearing, preferably, may be structured such that the retained austenite is in the range of 20% to 45%.
Still further, according to a fifth aspect of the invention, each thrust rolling bearing, preferably, may be structured such that the surface residual compressive stress thereof is set in the range of xe2x88x92200 to xe2x88x921800 MPa.
Yet further, according to a sixth aspect of the invention, each power roller, preferably, may be made of steel for machine-structural purposes having a carbon concentration in the range of 0.15-0.5% and the present steel may be carbonized or carbonitrided.
With use of the above-mentioned thrust rolling bearings, not only there can be obtained a toroidal-type continuously variable transmission having sufficient durability, but also a bearing of a stable quality can be obtained at a lower cost than the conventional bearing.